In many situations there is a need to establish the date on which a document was created and to prove that the text of a document in question is in fact the same as that of the original dated document. For example, in intellectual property matters it is often crucial to verify the date on which a person first put into writing the substance of an invention. A common procedure for thus "time-stamping" an inventive concept comprises daily notations of one's work in a laboratory notebook. Indelibly dated and signed entries are made one after another on each page of the notebook where the sequentially numbered, sewing pages make it difficult to revise the record without leaving telltale signs. The validity of the record is further enhanced by the regular review and signed witnessing by a generally disinterested third party. Should the time of the concept become a matter for later proof, both the physical substance of the notebook and the established recording procedure serve as effective evidence in substantiating the fact that the concept existed at least as early as the notebook witness date.
The increasingly widespread use of electronic documents, which include not only digital representations of readable text but also of video, audio, and pictorial data, now poses a serious threat to the viability of the "notebook" concept of establishing the date of any such document. Because electronic digital documents are so easily revised, and since such revisions may be made without telltale sign, there is available limited credible evidence that a given document truly states the date on which it was created or the message it originally carried. For the same reasons there even arises serious doubt as to the authenticity of a verifying signature. Without an effective procedure for ensuring against the surreptitious revision of digital documents, a basic lack of system credibility prevents the efficiencies of electronic documentation from being more widely implemented.
Some procedures are presently available for verifying electronic document transmissions; however, such procedures are limited in application to bilateral communications. That is, in such communications the sender essentially desires to verify to the receiver the source and original content of the transmitted document. For example, "private key" cryptographic schemes have long been employed for message transmission between or among a limited universe of individuals who are known to one another and who alone know the decrypting key. Encryption of the message ensures against tampering, and the fact that application of the private key reveals the "plaintext" of the transmitted message serves as proof that the message was transmitted by one of the defined universe. The time of creation of the message is only collaterally established, however, as being not later than its receipt by the addressee. This practice thus fails to provide time-stamp evidence that would be useful in an unlimited universe at a later date.
A more broadly applicable verifying communication procedure, that of "public key" cryptography, has been described by Diffie and Hellman ("New Directions in Cryptography", IEEE Transactions On Information Theory, Vol. IT-22, November 1976, pp. 644-654) and more recently implemented by Rivest et al. in U.S. Pat. No. 4,405,829, issued Sept. 20, 1983. While this scheme expands the utilizing universe to a substantially unlimited number of system subscribers who are unknown to one another, but for a public directory, verifiable communications remain bilateral. These limitations persist, since although a public key "signature", such as that which entails public key decryption of a message encrypted with the private key of the transmitter, provides any member of the unlimited universe with significant evidence of the identity of the transmitter of the message, only a given message recipient can be satisfied that the message existed at least as early as the time of its receipt. Such receipt does not, however, provide the whole universe with direct evidence of time of the message's existence. Testimony of such a recipient in conjunction with the received message could advance the proof of message content and time of its existence, but such evidence falls victim to the basic problem of ready manipulation of electronic digital document content, whether by originator or witness.
Thus, the prospect of a world in which all documents are in easily modifiable digital form threatens the very substance of existing procedures for establishing the credibility of such documents. There is clearly a significant present need for a system of verification by which a digital document may be so fixed in time and content that it can present, at least to the extent currently recognized in tangible documents, direct evidence on those issues.